It is known to provide a fuel injector with an injection nozzle, commonly referred to as a variable orifice nozzle (VON), in which a nozzle body is provided with a blind bore within which a first, outer valve is movable under the control of an actuator. The bore provided in the nozzle body defines a seating surface with which the outer valve is engageable to control fuel delivery through a first set of nozzle outlets provided at a first axial position along the main nozzle body axis. The outer valve is itself provided with a further bore within which a second, inner valve needle is able to move. The inner valve needle projects through the open end of the further bore in the outer valve and is engageable with the seating surface to control fuel delivery through a second set of nozzle outlets provided at a second, lower axial position along the nozzle body axis.
Variable orifice nozzles provide particular advantages for diesel engines, in that they provide the flexibility to inject fuel into the combustion chamber either through the first set of outlets on its own or through both the first and second outlets together. This enables selection of a fuel spray having a larger total fuel delivery area for high engine power modes or a smaller total fuel delivery area for lower engine power modes.
In a first injection stage the outer valve is operable to move alone, so that the outer valve is lifted away from its seating but the inner valve needle remains seated. In a second injection stage movement of the outer valve is transmitted to the inner valve needle, causing the inner valve needle to lift too, in circumstances in which the outer valve is moved through an amount exceeding a predetermined threshold amount. During this second injection stage, both the first and second sets of outlets are opened to give a relatively high fuel delivery rate. If the outer valve is lifted through an amount less than the predetermined threshold amount, the inner valve needle remains seated so that injection only occurs through the first set of outlets at a lower fuel delivery rate. An injection nozzle of this type is described in the Applicant's European patent 0967382 (Delphi Technologies Inc.), or in the Applicant's co-pending European patent application EP 04250132.0 (Delphi Technologies Inc.).
In an injector incorporating an injection nozzle of the aforementioned type, valve operation may be controlled by means of a piezoelectric actuator which allows fine control over the precise position of the outer valve by varying the extent to which the actuator is extended or contracted over the full actuator stroke. In previously proposed variable injection nozzles this has been a requirement to enable the nozzle to be switched between its first and second injection stages. Other types of actuator, however, such as electromagnetic actuators, do not allow such precise valve control so that their use with variable orifice nozzles has not previously been possible.
It is with a view to addressing this limitation that the present invention provides an improved injection nozzle.